"I, as president, swear upon my honour and integrity before the great Turkish nation and history to safeguard the existence and independence of the state," Erdogan said during a ceremony at the parliament in Ankara, which was broadcast live.
Erdogan received his mandate from the interim parliament Speaker before taking the oath.
Turkey's longest-serving leader, Erdogan won 52.2 per cent support in a May 28 runoff vote.
His election victory defied most opinion polls and came despite a cost-of-living crisis that was seen to have hurt his prospects.
His new five-year mandate allows Erdogan to pursue the increasingly authoritarian policies that have polarised the country, a NATO member, but strengthened its position as a regional military power.
After taking the oath in parliament, a ceremony will be held at the presidential palace attended by high-level officials from 78 countries and international organisations including NATO Secretary-General Jens Stoltenberg, Venezuelan President Nicolas Maduro, Hungarian Prime Minister Viktor Orban and Armenian Prime Minister Nikol Pashinyan, according to the state-run Anadolu Agency.
Erdogan will name his cabinet later on Saturday, and is expected to signal a change in his unorthodox approach to economic policy.
Erdogan was likely to include former economy chief Mehmet Simsek, Reuters reported earlier this week, which would indicate a potential return to greater economic orthodoxy including eventual interest rate hikes.
Simsek was highly regarded by investors when he served as finance minister and deputy prime minister between 2009 and 2018.
A key role for him now could mark a departure from years of sticking to low interest rates despite high inflation, and heavy state control of markets.
Erdogan, 69, became prime minister in 2003 after his AK Party won an election in late 2002 following Turkey's worst economic crisis since the 1970s.
In 2014, he became the country's first popularly elected president and was elected again in 2018 after securing new executive powers for the presidency in a 2017 referendum.
The May 14 presidential election and May 28 runoff were pivotal given that the opposition had been confident of ousting Erdogan and reversing many of his policies, including proposing sharp interest rate hikes to counter inflation, running at 44 per cent in April.
In his post-election victory speech, Erdogan said inflation, which hit a 24-year peak of 85 per cent last year before easing, was Turkey's most urgent issue.
Analysts have warned that if current policies continue, the economy is heading for turmoil given depleted foreign reserves, an expanding state-backed protected deposits scheme and unchecked inflation expectations.
The lira has undergone a series of crashes in recent years and hit new all-time lows in the days after the vote.
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